The present invention relates to an image formation device such as an electrophotographic copying machine and a laser printer, particularly relates to a multicolor image formation device for forming an image using a rotary developing method.
Heretofore, an image formation device for forming an electrostatic latent image by radiating a light beam modulated based upon digital image data on the surface of a photoconductor uniformly electrified, developing the electrostatic latent image and forming a toner image, transferring the toner image on recording paper, fixing and outputting the toner image on the recording paper with a fixing device is known.
Also, recently, a multicolor image formation device for forming a color image is rapidly being popularized. For the above multicolor image formation device, a multicolor image formation device which is provided with a rotary developing unit in which developing machines corresponding to each chromatic component such as cyan, magenta, yellow and black of a color image to be formed are arranged circularly and which uses a so-called rotary developing method for opposing each developing machine to a photoconductor on which an electrostatic latent image of each chromatic component is formed by sequentially switching the developing machines by rotating the rotary developing unit and developing by corresponding color toner is often utilized.
Further, for the multicolor image formation device using the rotary developing method, there is a type that an image is formed on recording paper by collectively transferring a final toner image from an intermediate transfer member onto the recording paper after color toner images of each chromatic component formed on a photoconductor are sequentially transferred on the intermediate transfer member and the final toner image is formed on the intermediate transfer member.
Heretofore, developing machines are switched as shown in a timing chart in FIG. 9. FIG. 9 shows time required for scanning a light beam on a photoconductor, time required for supplying toner onto the photoconductor by a developing machine to develop, time required for switching developing machines, time in which a developing machine is opposite to the photoconductor and time required for transferring a toner image formed on the photoconductor on an intermediate transfer member in order from the top.
For example, a developing machine for a second color, magenta (M) is switched soon after the developing of a first color, yellow (Y) is finished, similarly a developing machine for a third color, cyan (C) and a developing machine for a fourth color, black (K) are switched.
When developing machines are switched, impact is applied to a photoconductor because the developing roll of a developing machine and the photoconductor are touched in developing. As a color toner image formed on a photoconductor is transferred on an intermediate transfer member when developing machines are switched (at timing shown by a dotted line in FIG. 9), the defect of an image occurs due to impact applied to the photoconductor as shown by a point D in FIG. 11 in such a case. In FIG. 11, the horizontal axis shows time and the vertical axis shows the degree of impact applied to a photoconductor drum by %.
To prevent the above defect of an image from occurring, operation for switching developing machines is divided into two as shown in a timing chart in FIG. 10, a developing machine is once withdrawn after the developing of yellow for example is finished and the developing machine is switched to a developing machine for magenta immediately before the developing of magenta is started.
However, according to the above method of dividing operation for switching developing machines into two, the following another problem further occurs.
As shown in FIG. 12, an intermediate transfer belt 160 is wound on a driving roll 154 and plural tension rolls 156 and is turned in a direction shown by an arrow Q in FIG. 12 by the rotational driving of the driving roll 154. The intermediate transfer belt 160 and a photoconductor drum 150 are rotated at the same speed, however, difference in speed occurs due to dispersion in a diameter between the driving roll 154 and the photoconductor drum 150. As the photoconductor drum 150 and the intermediate transfer belt 160 easily adhere, deflection shown by a dotted line in FIG. 12 is caused between a primary transfer device 158 and the driving roll 154.
As the above deflection produces the similar effect to effect in case lubricant exists between the intermediate transfer belt 160 and the photoconductor 150 if toner is supplied to the photoconductor drum 150 by a developing machine 152Y, the intermediate transfer belt 160 and the photoconductor 150 slip and no deflection is caused. However, if the developing machine 152Y is once withdrawn to a position in which the developing machine 152Y is not in contact with the photoconductor 150 after the developing of yellow is finished and is switched to the developing machine 152M for magenta immediately before the developing of magenta is started as shown in the timing chart in FIG. 10, toner is not supplied to the photoconductor drum 150 during switching and the above deflection is caused.
When the developing machine 152Y is switched to the developing machine for magenta 152M and toner is again supplied to the photoconductor drum 150, tension is rapidly applied to the intermediate transfer belt 160 bent till the time, impact due to this shown at a point E in FIG. 11 causes the variation in speed of the photoconductor drum 150 and the defect of an image is caused.